The main theme, heard transformed in each movement of berlioz's symphonie fantastique, is called the

The main theme, heard transformed in each movement of berlioz's symphonie fantastique, is called the idée fixe. This term means 'a fixed idea,' and it refers to the main theme which occurs in each movement of this musical piece, although altered a bit each time it is heard. It is a structural foundation of the piece around which the plot is revolving. 

Ihaven't read it but i hope u find ya answer